It is known that long optical fiber transmission links for telecommunications can be built using cascaded chains of optical amplifiers. Erbium doped optical fiber amplifiers are particularly well-suited for implementing these long distance transmission systems due to their excellent performance characteristics and ease of fabrication.
However, multiplexed optical- signals utilizing wavelength division multiplexed (WDM) systems and erbium doped optical amplifiers exhibit a variation in signal gain that is a function of the individual wavelengths. Moreover, utilizing cascaded optical amplifiers to compensate for attenuation over the transmission link only exaggerates the variation in signal gain for the separate wavelengths. For example, a 10 channel WDM system with a 1 nm channel spacing could easily have a gain variation over the 10 nm signal band of from 1 to 3 dB after amplification. The total gain variation is increased by the product of the number of cascaded amplifiers, and thus will certainly be much larger. While a 1 to 3 dB gain variation may be acceptable for short amplifier chains, with 10 or more cascaded amplifiers the resulting 10 to 30 dB gain variation is not likely to be acceptable.
Large variation in component signal levels of a multiplexed signal over the wavelength spectrum complicates the design and performance of optical receivers and -detectors, and thus it is advantageous to equalize variation in signal level for any wavelength-dependent elements in the optical transmission path, particularly wavelength-dependent gain due to amplification.